A currently used photodiode amplifier is shown in FIG. 1. A photodiode 10 is connected across the input terminals of an amplifier 11 between a voltage reference 12 and a feedback resistor 13. The frequency response of such a circuit is determined by the resistance R.sub.F of the feedback resistor 13 and the capacitance C of the photodiode 10. The bandwidth of the amplifer 11 will be the gain-bandwidth product divided by the gain. Consequently, at high gains the bandwidth is small. More specifically, because the open loop gain of the amplifier 11 will be diminished at high frequencies, the effective impedance seen by the photodiode 10 will be large. Thus, the circuit performs with a low pass characteristic, having a cutoff frequency f.sub.c1/2.pi.R.sub.I C, where R.sub.I is the effective input impedance of the circuit. Unfortunately, the choices for R.sub.F and C are not optimal in relation to broadband usage, since the photodiode 10 must be large (which generates a large capacitance C) in order to obtain sufficient light sensitivity while the amplifier 11 requires a large feedback impedance (resistance R.sub.F of the feedback resistor 13) in order to obtain sufficient transimpedance gain. Reducing either value defeats the purpose of the circuit. As a result, the cutoff frequency fc is ordinarily close to DC values, and high frequency waveforms are not output by the amplifier 11. This becomes a particular problem in photographic applications where the output of the photodiode amplifier is intended for use in discriminating the type of illuminant.
To produce faithful photographic reproductions of multicolored scenes, the color balance of the photographic film must be compatible with the spectral characteristics of the scene illuminant. Many photographic color emulsions are color balanced for use with natural daylight and others are color balanced for use with incandescent illumination. To properly expose a color film with an illuminant for which the film is not color balanced it is necessary to use color compensating filters. Alternatively, correction can be made during the printing stage. When such compensation is automatically provided by the camera, by engaging the proper filter or by marking the film with a printing instruction, it is necessary to have some automatic technique for discriminating among various types of light sources.
A patent of interest for its teaching of a method and apparatus for discriminating illuminant light is U.S. Pat. No. 4,220,412, entitled "Illuminant Discrimination Apparatus and Method" by R. A. Shroyer, et al. The method and apparatus disclosed in that patent utilizes the peak amplitude and the harmonic distortion in the sine wave signal that is derived from the illuminant source impinging on a photodiode. In particular, the flicker ratio, which is the ratio of the brightest to the dimmest intensities of the light during a given time interval, is detected. Natural light, like other light emanating from a source of constant brightness, has a flicker ratio of unity. Artificial light sources, being energized by ordinary household line voltage, have a brightness which flickers at approximately 120 Hz, twice the frequency of the line voltage. Owing to the different rates at which the energy-responsive elements of incandescent and fluorescent lamps respond to applied energy, such illuminance can be readily distinguished by their respective flicker ratio. Using the harmonic content, it is further possible to distinguish incandescent light from fluorescent light mixed with daylight and to detect which light source is predominant in a mixture of fluorescent and incandescent light.
With the general interest in digital systems, it is useful to incorporate illuminant discrimination into a digital environment. This is done in U.S. Pat. No. 4,827,119, which discriminates among various types of illuminants such as fluorescent light, tungsten light and natural daylight. The apparatus utilizes an analog-to-digital converter and a microprocessor to perform a Fourier series analysis on one or more of the harmonics of the digitized illuminant signal. The microprocessor compares the amplitudes of the harmonics against the amplitudes of known illuminant sources to identify the source.
In certain situations it is desirable to separate scenes having a dominant illuminant from scenes having mixed illumination with no single dominant illuminant. In such cases, color correction is best handled by printing algorithms. In U.S. Pat. No. 5,037,198, mixed illuminant detection is obtained by using boundary conditions in order to eliminate detection errors seen when fluorescent illumination mixes with certain quantities of daylight and otherwise causes a tungsten reading. The electrical signals derived from the illuminant are processed in a log amplifier to form a signal approximately equal to the log of the DC term plus a ratio of the dominant AC components to the DC components. The signals output from the log amplifier are converted into two filtered outputs which are multiples of the frequency of expected artificial illumination sources. Each of the output signals is compared against a plurality of threshold signals to identify which illuminant components are present, and to identify mixed sources.
The methods employed in U.S. Pat. Nos. 4,827,119 and 5,037,198 examine the frequency of flicker in the light intensity spectrum and determine from the frequency harmonics which type of illumination is being used. A problem has arisen because new fluorescent lighting systems use power inverters to increase the frequency of operation and the efficiency of the light sources. With such high efficiency fluorescent illumination, it is difficult to detect these higher frequencies, which are as high as 70 kHz, due to speed limitations in the circuit topologies typically used in amplification of the signal generated by the photodiode detector.